The present invention is directed to structural aircraft components composed of cast INCONEL(copyright) 718 and forged WASPALOY(trademark) or cast INCONEL(copyright) 718 and forged INCOLOY(copyright) 718/903/907/909, among others.
Many structural aircraft engine components are made of a combination of either solid cast INCONEL(copyright) 718 or cast INCONEL(copyright) 718 and a separate forged component. INCONEL(copyright) is a registered trademark of Huntington Alloys Corporation of Huntington, W. Va. The separate forged component is usually a material such as forged INCONEL(copyright) 718, forged WASPALOY(trademark), or forged INCOLOY(copyright) 903/907/909, among others. WASPALOY(trademark) is an unregistered trademark of Haynes International, Inc. of Kokomo, Ind. INCOLOY(copyright) is a registered trademark of Inco Alloys International, Inc. of Huntington, W. Va. These materials are commonly joined as an inseparable assembly by welding them together. During engine operation, these components may develop cracking in one of the materials rendering the component non-serviceable.
Cast INCONEL(copyright) 718 is a nickel based superalloy that obtains its desirable properties by precipitation hardening at an elevated temperature. INCONEL(copyright) 718 is a well-known trademark for a nickel-based superalloy having a nominal composition, in weight percent, of about 18.5 percent iron, about 18.5 percent chromium, about 5.1 percent niobium, about 3 percent molybdenum, about 0.9 percent titanium, about 0.5 percent aluminum, about 0.04 percent carbon, and balance nickel, which composition is well-known to those skilled in the art. Both the cast INCONEL(copyright) 718 and the associated wrought structures have the desirable physical properties of warm temperature strength, creep strength, stress rupture strength, and fatigue resistance, for application of the article as a high temperature engine aircraft structural component. In order to obtain these desirable properties, both the cast INCONEL(copyright) 718 and the associated wrought structures require a proper amount of the gamma-prime (xcex3xe2x80x2) phase and the gamma-double-prime (xcex3xe2x80x3) phase. The xcex3xe2x80x3 phase, which is a body-centered tetragonal precipitate in a simple face-centered cubic structure, is metastable and forms an undesirable phase, the delta phase (xcex4), in the temperature range of 1200xc2x0 F. to 1800xc2x0 F. The xcex4 phase nucleates at the grain boundaries of the cast INCONEL(copyright) 718 and the associated wrought structures at the expense of xcex3xe2x80x3 which xcex4 phase coarsens rapidly unless it is solutioned at elevated temperatures. The presence of xcex4 leads to the degradation of both weldability and the mechanical properties of the cast INCONEL(copyright) 718 and the associated wrought structure.
A method for repairing these cracks is generally found in engine maintenance manuals, which allow the components to be repaired and returned to serviceable condition Typically, these repair methods consist of welding the cracks in order to heal them, followed by a stress relief beat treatment. For cast INCONEL(copyright) 718 with forged attachment parts the repair process consists of pre-heating the assembly at about 1750xc2x0 F. for about one hour, post weld heat treating at about 1750xc2x0 F. for one hour, followed by an aging heat treatment to form xcex3xe2x80x3.
The aerospace structural components employing cast INCONEL(copyright) 718 are not life limited. Such structural components have no planted time for their obsolescence. Included in these components are major aircraft engine frames, cases and supports that are inspected at certain durations of time and or cycles of the engine. If non-serviceable conditions are found during these inspections, then the non-conforming components are disassembled from the engine and sent to a repair shop. This is commonly called a xe2x80x9cshop visitxe2x80x9d.
It is not uncommon to find cracking on INCONEL(copyright) 718 components that require the standard weld and heat treat repair during shop visits as set forth above. Such visits cause multiple generations of weld and heat treat repairs. These multi-generational repairs cause degradation of the cast INCONEL(copyright) 718 material due to the formation of xcex4 phase precipitates over time. Data from several repair stations show that the effectiveness of the weld/heat repairs decrease proportionally with the frequency of these repairs. For example on the CF6-50 Compressor Rear Frame, one airline reports that the frame will be operated on an engine for an average of 25,000 hours before a crack appears at the bleed ports at the end of the struts. After the crack is repaired by performing known local weld/heat treat repair processes, and the frame is returned to service, a new crack will appear in the area of the bleed port near the weld/heat treat repair. The average time for a new crack to appear is 5,000 hours after the original repair. Therefore, if the time it takes for a crack to appear from the time the new frame is placed in service is about 25,000 hours, then the time it takes a new crack to appear after a weld and heat treat repair is about 20% of the original service time. This is just one example of many reports from different airlines.
The primary cause of the reduced service usage (crack free) of the frames after repair is the degradation of the cast INCONEL(copyright) 718 material. Repeated heating and cooling cycles in the temperature range of 1700xc2x0 F. to 1800xc2x0 F. causes formation of the xcex4 phase. The material accumulates delta phase material from the weld and heat treat repair, which is exacerbated with multiple cycles. The presence of this delta phase indicates that the distribution of certain key elements in the alloy is altered in such a way that elements have collectively migrated to certain areas where they are now highly concentrated. This depletes these elements from other areas, decreasing the mechanical properties of the alloy in these areas. Therefore, key elements must be redistributed properly in the alloy to prevent cracking, since the mechanical properties of cast INCONEL(copyright) 718 are decreased when xcex4 is present.
The present invention is directed toward improvements in the repair and heat treatments used to restore cast INCONEL(copyright) 718 aircraft engine parts to provide a more uniform distribution of elements. Over time, and after numerous crack repairs and heat treatments, the mechanical properties of cast INCONEL(copyright) 718 deteriorates. The process of the present invention allows the restoration of cast INCONEL(copyright) 718 to a state which is similar to the condition of the cast INCONEL(copyright) 718 immediately after manufacture.
The article, which includes a cast INCONEL(copyright) 718 component is restored through a process that includes beat treatment. First, the article that typically includes a cast portion and a forged portion is placed into a heat treatment chamber, purged of oxygen and the pressure in the chamber is set to a suitable neutral or reducing atmosphere. The article is then heated, at a rate suitable to minimize distortion, to a temperature in the range of about 1950xc2x0 F. to about 215 xc2x0 F. The temperature of the article is then held in a range of about 1950xc2x0 F. to about 2150xc2x0 F. for a time sufficient to solutionize the delta phase precipitates and homogenize the alloy. The article is then cooled at a rate sufficient to avoid delta phase precipitation in the range of about 1600xc2x0 F. to about 1900xc2x0 F. in a protective neutral or reducing atmosphere at a rate sufficient to maintain dimensional stability. The article should then be air quenched, or quenched in an inert gas at an equivalent rate, to room temperature. The forged portion can then be removed, leaving a cast portion that has essentially a solutioned condition. As used herein, the terms xe2x80x9cwroughtxe2x80x9d and xe2x80x9cforgedxe2x80x9d are used interchangeably. The cast portion can then be reused, while the wrought portion is disgarded
Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings which illustrate, by way of example, the principles of the invention.